diff --git a/cmd/promtool/tsdb.go b/cmd/promtool/tsdb.go
index 80b1dc42ff..36e33049d1 100644
--- a/cmd/promtool/tsdb.go
+++ b/cmd/promtool/tsdb.go
@@ -469,16 +469,15 @@ func analyzeBlock(path, blockID string, limit int, runExtended bool) error {
 	if err != nil {
 		return err
 	}
-	lbls := labels.Labels{}
 	chks := []chunks.Meta{}
 	builder := labels.ScratchBuilder{}
 	for p.Next() {
-		if err = ir.Series(p.At(), &builder, &lbls, &chks); err != nil {
+		if err = ir.Series(p.At(), &builder, &chks); err != nil {
 			return err
 		}
 		// Amount of the block time range not covered by this series.
 		uncovered := uint64(meta.MaxTime-meta.MinTime) - uint64(chks[len(chks)-1].MaxTime-chks[0].MinTime)
-		lbls.Range(func(lbl labels.Label) {
+		builder.Labels().Range(func(lbl labels.Label) {
 			key := lbl.Name + "=" + lbl.Value
 			labelsUncovered[lbl.Name] += uncovered
 			labelpairsUncovered[key] += uncovered
@@ -591,9 +590,8 @@ func analyzeCompaction(block tsdb.BlockReader, indexr tsdb.IndexReader) (err err
 	totalChunks := 0
 	var builder labels.ScratchBuilder
 	for postingsr.Next() {
-		lbsl := labels.Labels{}
 		var chks []chunks.Meta
-		if err := indexr.Series(postingsr.At(), &builder, &lbsl, &chks); err != nil {
+		if err := indexr.Series(postingsr.At(), &builder, &chks); err != nil {
 			return err
 		}
 
diff --git a/model/labels/labels.go b/model/labels/labels.go
index 453c3f60dd..36a0e6cb35 100644
--- a/model/labels/labels.go
+++ b/model/labels/labels.go
@@ -592,15 +592,14 @@ func (b *ScratchBuilder) Sort() {
 	sort.Sort(b.add)
 }
 
+// Asssign is for when you already have a Labels which you want this ScratchBuilder to return.
+func (b *ScratchBuilder) Assign(ls Labels) {
+	b.add = append(b.add[:0], ls...) // Copy on top of our slice, so we don't retain the input slice.
+}
+
 // Return the name/value pairs added so far as a Labels object.
 // Note: if you want them sorted, call Sort() first.
 func (b *ScratchBuilder) Labels() Labels {
 	// Copy the slice, so the next use of ScratchBuilder doesn't overwrite.
 	return append([]Label{}, b.add...)
 }
-
-// Write the newly-built Labels out to ls, reusing its buffer if long enough.
-// Callers must ensure that there are no other references to ls.
-func (b *ScratchBuilder) Overwrite(ls *Labels) {
-	(*ls) = append((*ls)[:0], b.add...)
-}
diff --git a/model/labels/labels_test.go b/model/labels/labels_test.go
index 3a1732d22a..69beb2fc3e 100644
--- a/model/labels/labels_test.go
+++ b/model/labels/labels_test.go
@@ -622,7 +622,6 @@ func TestScratchBuilder(t *testing.T) {
 			want: FromStrings("ddd", "444"),
 		},
 	} {
-		overwriteTarget := EmptyLabels()
 		t.Run(fmt.Sprint(i), func(t *testing.T) {
 			b := ScratchBuilder{}
 			for _, lbl := range tcase.add {
@@ -630,8 +629,8 @@ func TestScratchBuilder(t *testing.T) {
 			}
 			b.Sort()
 			require.Equal(t, tcase.want, b.Labels())
-			b.Overwrite(&overwriteTarget)
-			require.Equal(t, tcase.want, overwriteTarget)
+			b.Assign(tcase.want)
+			require.Equal(t, tcase.want, b.Labels())
 		})
 	}
 }
diff --git a/tsdb/block.go b/tsdb/block.go
index 33413bb21a..401ab59e0a 100644
--- a/tsdb/block.go
+++ b/tsdb/block.go
@@ -79,10 +79,10 @@ type IndexReader interface {
 	// by the label set of the underlying series.
 	SortedPostings(index.Postings) index.Postings
 
-	// Series populates the given labels and chunk metas for the series identified
+	// Series populates the given builder and chunk metas for the series identified
 	// by the reference.
 	// Returns storage.ErrNotFound if the ref does not resolve to a known series.
-	Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error
+	Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error
 
 	// LabelNames returns all the unique label names present in the index in sorted order.
 	LabelNames(matchers ...*labels.Matcher) ([]string, error)
@@ -499,8 +499,8 @@ func (r blockIndexReader) SortedPostings(p index.Postings) index.Postings {
 	return r.ir.SortedPostings(p)
 }
 
-func (r blockIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
-	if err := r.ir.Series(ref, builder, lset, chks); err != nil {
+func (r blockIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
+	if err := r.ir.Series(ref, builder, chks); err != nil {
 		return errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 	}
 	return nil
@@ -561,13 +561,12 @@ func (pb *Block) Delete(mint, maxt int64, ms ...*labels.Matcher) error {
 	// Choose only valid postings which have chunks in the time-range.
 	stones := tombstones.NewMemTombstones()
 
-	var lset labels.Labels
 	var chks []chunks.Meta
 	var builder labels.ScratchBuilder
 
 Outer:
 	for p.Next() {
-		err := ir.Series(p.At(), &builder, &lset, &chks)
+		err := ir.Series(p.At(), &builder, &chks)
 		if err != nil {
 			return err
 		}
diff --git a/tsdb/db_test.go b/tsdb/db_test.go
index 1d95cfe00e..aeb5fa4bee 100644
--- a/tsdb/db_test.go
+++ b/tsdb/db_test.go
@@ -1843,15 +1843,12 @@ func TestChunkAtBlockBoundary(t *testing.T) {
 		p, err := r.Postings(k, v)
 		require.NoError(t, err)
 
-		var (
-			lset labels.Labels
-			chks []chunks.Meta
-		)
+		var chks []chunks.Meta
 
 		chunkCount := 0
 
 		for p.Next() {
-			err = r.Series(p.At(), &builder, &lset, &chks)
+			err = r.Series(p.At(), &builder, &chks)
 			require.NoError(t, err)
 			for _, c := range chks {
 				require.True(t, meta.MinTime <= c.MinTime && c.MaxTime <= meta.MaxTime,
diff --git a/tsdb/head_read.go b/tsdb/head_read.go
index 0a6aef2bd2..464665ae87 100644
--- a/tsdb/head_read.go
+++ b/tsdb/head_read.go
@@ -148,14 +148,14 @@ func (h *headIndexReader) SortedPostings(p index.Postings) index.Postings {
 }
 
 // Series returns the series for the given reference.
-func (h *headIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
+func (h *headIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	s := h.head.series.getByID(chunks.HeadSeriesRef(ref))
 
 	if s == nil {
 		h.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	lbls.CopyFrom(s.lset)
+	builder.Assign(s.lset)
 
 	s.Lock()
 	defer s.Unlock()
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index cc0133b9d5..16800d75e2 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -1452,13 +1452,12 @@ func TestGCChunkAccess(t *testing.T) {
 
 	idx := h.indexRange(0, 1500)
 	var (
-		lset    labels.Labels
 		chunks  []chunks.Meta
 		builder labels.ScratchBuilder
 	)
-	require.NoError(t, idx.Series(1, &builder, &lset, &chunks))
+	require.NoError(t, idx.Series(1, &builder, &chunks))
 
-	require.Equal(t, labels.FromStrings("a", "1"), lset)
+	require.Equal(t, labels.FromStrings("a", "1"), builder.Labels())
 	require.Equal(t, 2, len(chunks))
 
 	cr, err := h.chunksRange(0, 1500, nil)
@@ -1506,13 +1505,12 @@ func TestGCSeriesAccess(t *testing.T) {
 
 	idx := h.indexRange(0, 2000)
 	var (
-		lset    labels.Labels
 		chunks  []chunks.Meta
 		builder labels.ScratchBuilder
 	)
-	require.NoError(t, idx.Series(1, &builder, &lset, &chunks))
+	require.NoError(t, idx.Series(1, &builder, &chunks))
 
-	require.Equal(t, labels.FromStrings("a", "1"), lset)
+	require.Equal(t, labels.FromStrings("a", "1"), builder.Labels())
 	require.Equal(t, 2, len(chunks))
 
 	cr, err := h.chunksRange(0, 2000, nil)
diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index 9cf2425ece..407b5380fa 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -1596,8 +1596,8 @@ func (r *Reader) LabelValueFor(id storage.SeriesRef, label string) (string, erro
 	return value, nil
 }
 
-// Series reads the series with the given ID and writes its labels and chunks into lbls and chks.
-func (r *Reader) Series(id storage.SeriesRef, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
+// Series reads the series with the given ID and writes its labels and chunks into builder and chks.
+func (r *Reader) Series(id storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	offset := id
 	// In version 2 series IDs are no longer exact references but series are 16-byte padded
 	// and the ID is the multiple of 16 of the actual position.
@@ -1608,7 +1608,7 @@ func (r *Reader) Series(id storage.SeriesRef, builder *labels.ScratchBuilder, lb
 	if d.Err() != nil {
 		return d.Err()
 	}
-	return errors.Wrap(r.dec.Series(d.Get(), builder, lbls, chks), "read series")
+	return errors.Wrap(r.dec.Series(d.Get(), builder, chks), "read series")
 }
 
 func (r *Reader) Postings(name string, values ...string) (Postings, error) {
@@ -1835,9 +1835,9 @@ func (dec *Decoder) LabelValueFor(b []byte, label string) (string, error) {
 	return "", d.Err()
 }
 
-// Series decodes a series entry from the given byte slice into lset and chks.
+// Series decodes a series entry from the given byte slice into builder and chks.
 // Previous contents of lbls can be overwritten - make sure you copy before retaining.
-func (dec *Decoder) Series(b []byte, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
+func (dec *Decoder) Series(b []byte, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	builder.Reset()
 	*chks = (*chks)[:0]
 
@@ -1864,7 +1864,6 @@ func (dec *Decoder) Series(b []byte, builder *labels.ScratchBuilder, lbls *label
 
 		builder.Add(ln, lv)
 	}
-	builder.Overwrite(lbls)
 
 	// Read the chunks meta data.
 	k = d.Uvarint()
diff --git a/tsdb/index/index_test.go b/tsdb/index/index_test.go
index e6b2e890c5..e4cee4a55d 100644
--- a/tsdb/index/index_test.go
+++ b/tsdb/index/index_test.go
@@ -124,12 +124,12 @@ func (m mockIndex) SortedPostings(p Postings) Postings {
 	return NewListPostings(ep)
 }
 
-func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	s, ok := m.series[ref]
 	if !ok {
 		return errors.New("not found")
 	}
-	lset.CopyFrom(s.l)
+	builder.Assign(s.l)
 	*chks = append((*chks)[:0], s.chunks...)
 
 	return nil
@@ -197,16 +197,15 @@ func TestIndexRW_Postings(t *testing.T) {
 	p, err := ir.Postings("a", "1")
 	require.NoError(t, err)
 
-	var l labels.Labels
 	var c []chunks.Meta
 	var builder labels.ScratchBuilder
 
 	for i := 0; p.Next(); i++ {
-		err := ir.Series(p.At(), &builder, &l, &c)
+		err := ir.Series(p.At(), &builder, &c)
 
 		require.NoError(t, err)
 		require.Equal(t, 0, len(c))
-		require.Equal(t, series[i], l)
+		require.Equal(t, series[i], builder.Labels())
 	}
 	require.NoError(t, p.Err())
 
@@ -318,11 +317,10 @@ func TestPostingsMany(t *testing.T) {
 		require.NoError(t, err)
 
 		got := []string{}
-		var lbls labels.Labels
 		var metas []chunks.Meta
 		for it.Next() {
-			require.NoError(t, ir.Series(it.At(), &builder, &lbls, &metas))
-			got = append(got, lbls.Copy().Get("i"))
+			require.NoError(t, ir.Series(it.At(), &builder, &metas))
+			got = append(got, builder.Labels().Get("i"))
 		}
 		require.NoError(t, it.Err())
 		exp := []string{}
@@ -421,21 +419,20 @@ func TestPersistence_index_e2e(t *testing.T) {
 		expp, err := mi.Postings(p.Name, p.Value)
 		require.NoError(t, err)
 
-		var lset, explset labels.Labels
 		var chks, expchks []chunks.Meta
-		var builder labels.ScratchBuilder
+		var builder, eBuilder labels.ScratchBuilder
 
 		for gotp.Next() {
 			require.True(t, expp.Next())
 
 			ref := gotp.At()
 
-			err := ir.Series(ref, &builder, &lset, &chks)
+			err := ir.Series(ref, &builder, &chks)
 			require.NoError(t, err)
 
-			err = mi.Series(expp.At(), &builder, &explset, &expchks)
+			err = mi.Series(expp.At(), &eBuilder, &expchks)
 			require.NoError(t, err)
-			require.Equal(t, explset, lset)
+			require.Equal(t, eBuilder.Labels(), builder.Labels())
 			require.Equal(t, expchks, chks)
 		}
 		require.False(t, expp.Next(), "Expected no more postings for %q=%q", p.Name, p.Value)
diff --git a/tsdb/ooo_head_read.go b/tsdb/ooo_head_read.go
index d0b09963d7..3486083f26 100644
--- a/tsdb/ooo_head_read.go
+++ b/tsdb/ooo_head_read.go
@@ -47,21 +47,21 @@ func NewOOOHeadIndexReader(head *Head, mint, maxt int64) *OOOHeadIndexReader {
 	return &OOOHeadIndexReader{hr}
 }
 
-func (oh *OOOHeadIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lbls *labels.Labels, chks *[]chunks.Meta) error {
-	return oh.series(ref, lbls, chks, 0)
+func (oh *OOOHeadIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
+	return oh.series(ref, builder, chks, 0)
 }
 
 // The passed lastMmapRef tells upto what max m-map chunk that we can consider.
 // If it is 0, it means all chunks need to be considered.
 // If it is non-0, then the oooHeadChunk must not be considered.
-func (oh *OOOHeadIndexReader) series(ref storage.SeriesRef, lbls *labels.Labels, chks *[]chunks.Meta, lastMmapRef chunks.ChunkDiskMapperRef) error {
+func (oh *OOOHeadIndexReader) series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta, lastMmapRef chunks.ChunkDiskMapperRef) error {
 	s := oh.head.series.getByID(chunks.HeadSeriesRef(ref))
 
 	if s == nil {
 		oh.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	lbls.CopyFrom(s.lset)
+	builder.Assign(s.lset)
 
 	if chks == nil {
 		return nil
@@ -400,8 +400,8 @@ func (ir *OOOCompactionHeadIndexReader) SortedPostings(p index.Postings) index.P
 	return p
 }
 
-func (ir *OOOCompactionHeadIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
-	return ir.ch.oooIR.series(ref, lset, chks, ir.ch.lastMmapRef)
+func (ir *OOOCompactionHeadIndexReader) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
+	return ir.ch.oooIR.series(ref, builder, chks, ir.ch.lastMmapRef)
 }
 
 func (ir *OOOCompactionHeadIndexReader) SortedLabelValues(name string, matchers ...*labels.Matcher) ([]string, error) {
diff --git a/tsdb/ooo_head_read_test.go b/tsdb/ooo_head_read_test.go
index c48e95e465..4448443c44 100644
--- a/tsdb/ooo_head_read_test.go
+++ b/tsdb/ooo_head_read_test.go
@@ -357,14 +357,13 @@ func TestOOOHeadIndexReader_Series(t *testing.T) {
 					ir := NewOOOHeadIndexReader(h, tc.queryMinT, tc.queryMaxT)
 
 					var chks []chunks.Meta
-					var respLset labels.Labels
 					var b labels.ScratchBuilder
-					err := ir.Series(storage.SeriesRef(s1ID), &b, &respLset, &chks)
+					err := ir.Series(storage.SeriesRef(s1ID), &b, &chks)
 					require.NoError(t, err)
-					require.Equal(t, s1Lset, respLset)
+					require.Equal(t, s1Lset, b.Labels())
 					require.Equal(t, expChunks, chks)
 
-					err = ir.Series(storage.SeriesRef(s1ID+1), &b, &respLset, &chks)
+					err = ir.Series(storage.SeriesRef(s1ID+1), &b, &chks)
 					require.Equal(t, storage.ErrNotFound, err)
 				})
 			}
@@ -841,9 +840,8 @@ func TestOOOHeadChunkReader_Chunk(t *testing.T) {
 			// markers like OOOLastRef. These are then used by the ChunkReader.
 			ir := NewOOOHeadIndexReader(db.head, tc.queryMinT, tc.queryMaxT)
 			var chks []chunks.Meta
-			var respLset labels.Labels
 			var b labels.ScratchBuilder
-			err := ir.Series(s1Ref, &b, &respLset, &chks)
+			err := ir.Series(s1Ref, &b, &chks)
 			require.NoError(t, err)
 			require.Equal(t, len(tc.expChunksSamples), len(chks))
 
@@ -1005,9 +1003,8 @@ func TestOOOHeadChunkReader_Chunk_ConsistentQueryResponseDespiteOfHeadExpanding(
 			// markers like OOOLastRef. These are then used by the ChunkReader.
 			ir := NewOOOHeadIndexReader(db.head, tc.queryMinT, tc.queryMaxT)
 			var chks []chunks.Meta
-			var respLset labels.Labels
 			var b labels.ScratchBuilder
-			err := ir.Series(s1Ref, &b, &respLset, &chks)
+			err := ir.Series(s1Ref, &b, &chks)
 			require.NoError(t, err)
 			require.Equal(t, len(tc.expChunksSamples), len(chks))
 
diff --git a/tsdb/querier.go b/tsdb/querier.go
index 34917dad0a..7b3ba8e4be 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -451,14 +451,13 @@ type blockBaseSeriesSet struct {
 	curr seriesData
 
 	bufChks []chunks.Meta
-	bufLbls labels.Labels
 	builder labels.ScratchBuilder
 	err     error
 }
 
 func (b *blockBaseSeriesSet) Next() bool {
 	for b.p.Next() {
-		if err := b.index.Series(b.p.At(), &b.builder, &b.bufLbls, &b.bufChks); err != nil {
+		if err := b.index.Series(b.p.At(), &b.builder, &b.bufChks); err != nil {
 			// Postings may be stale. Skip if no underlying series exists.
 			if errors.Cause(err) == storage.ErrNotFound {
 				continue
@@ -529,7 +528,7 @@ func (b *blockBaseSeriesSet) Next() bool {
 			intervals = intervals.Add(tombstones.Interval{Mint: b.maxt + 1, Maxt: math.MaxInt64})
 		}
 
-		b.curr.labels = b.bufLbls.Copy()
+		b.curr.labels = b.builder.Labels()
 		b.curr.chks = chks
 		b.curr.intervals = intervals
 
diff --git a/tsdb/querier_test.go b/tsdb/querier_test.go
index eb0c847fd1..e6e9f143f1 100644
--- a/tsdb/querier_test.go
+++ b/tsdb/querier_test.go
@@ -1270,12 +1270,12 @@ func (m mockIndex) SortedPostings(p index.Postings) index.Postings {
 	return index.NewListPostings(ep)
 }
 
-func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	s, ok := m.series[ref]
 	if !ok {
 		return storage.ErrNotFound
 	}
-	lset.CopyFrom(s.l)
+	builder.Assign(s.l)
 	*chks = append((*chks)[:0], s.chunks...)
 
 	return nil
@@ -1886,8 +1886,8 @@ func TestPostingsForMatchers(t *testing.T) {
 
 		var builder labels.ScratchBuilder
 		for p.Next() {
-			lbls := labels.Labels{}
-			require.NoError(t, ir.Series(p.At(), &builder, &lbls, &[]chunks.Meta{}))
+			require.NoError(t, ir.Series(p.At(), &builder, &[]chunks.Meta{}))
+			lbls := builder.Labels()
 			if _, ok := exp[lbls.String()]; !ok {
 				t.Errorf("Evaluating %v, unexpected result %s", c.matchers, lbls.String())
 			} else {
@@ -2101,7 +2101,7 @@ func (m mockMatcherIndex) SortedPostings(p index.Postings) index.Postings {
 	return index.EmptyPostings()
 }
 
-func (m mockMatcherIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, lset *labels.Labels, chks *[]chunks.Meta) error {
+func (m mockMatcherIndex) Series(ref storage.SeriesRef, builder *labels.ScratchBuilder, chks *[]chunks.Meta) error {
 	return nil
 }
 
diff --git a/tsdb/repair_test.go b/tsdb/repair_test.go
index d394115601..d4e9b76ad0 100644
--- a/tsdb/repair_test.go
+++ b/tsdb/repair_test.go
@@ -84,8 +84,7 @@ func TestRepairBadIndexVersion(t *testing.T) {
 	for p.Next() {
 		t.Logf("next ID %d", p.At())
 
-		var lset labels.Labels
-		require.Error(t, r.Series(p.At(), &builder, &lset, nil))
+		require.Error(t, r.Series(p.At(), &builder, nil))
 	}
 	require.NoError(t, p.Err())
 	require.NoError(t, r.Close())
@@ -105,10 +104,9 @@ func TestRepairBadIndexVersion(t *testing.T) {
 	for p.Next() {
 		t.Logf("next ID %d", p.At())
 
-		var lset labels.Labels
 		var chks []chunks.Meta
-		require.NoError(t, r.Series(p.At(), &builder, &lset, &chks))
-		res = append(res, lset)
+		require.NoError(t, r.Series(p.At(), &builder, &chks))
+		res = append(res, builder.Labels())
 	}
 
 	require.NoError(t, p.Err())